Turbomachinery, such as steam and gas turbines, include a plurality of blades arranged in rows extending radially from an axially aligned shaft, the rows of blades being rotatable in response to the force of a high pressure fluid flowing axially through the machine. Due to their complex design the blades have many resonant vibrational frequencies which may be reinforced by blade rotational speeds or harmonics thereof. Blade resonant frequencies which are reinforced by rotational speeds may create stresses sufficient to break the blades and cause extensive damage. Although blades are designed and tested prior to machine installation in order to prevent resonant vibration, such evaluations performed prior to actual use do not subject the blades to the same temperature, pressure and rotational conditions which are experienced during normal operations. Consequently, it is desirable to monitor rotating blades on-line in order to detect all resonant vibrations. It is also desirable to monitor rotating blades on-line in order to detect new vibration problems which develop after a turbomachine is put in use. New blade vibrations may be indicative of significant structural changes which may also lead to extensive damage as well as costly down time while the machine undergoes repair.
Vibration monitoring systems have been developed for on-line detection of blade vibrations. A typical system may employ as many as twenty-four noncontacting proximity sensors concentrically mounted about individual blade rows. Each sensor is used to detect motion of a rotating blade tip about its normal position in a rotating time frame. Sensors are generally of the magnetic induction type which develop a small electrical output as a blade tip rotates past it. Many vibration monitoring systems measure the time required for a blade tip to travel between two reference points which are separated by a known distance. Deviations between the measured travel time of the blade tip and the expected travel time based on shaft rotational speed are calculated. The deviations are used to reconstruct a time history of vibrational movement for each blade tip. Fourier analysis is then used to determine the amplitudes and frequencies of the vibration present in each blade.
Because measurement of blade tip vibration is based on position-time data, sensor misalignment will result in distortion of the reconstructed vibration wave forms. Therefore, the vibration sensors are precision mounted to a large ring which is positioned along the turbine casing, radially outward from a blade row. In the past, the dimensions of this ring and the precision machining required for aligning the vibration sensors on the ring have formed a large part of the production and installation costs of vibration monitoring systems for turbomachinery. Furthermore, relatively small errors in the machining process may significantly affect the relative positioning of sensors and require that a machine ring be discarded. It is desirable to have a less costly means for mounting the vibration sensors which also provides for readjustment of sensor alignment. Such an adjustable mounting mechanism may also be useful for correcting sensor misalignments which result from thermal expansion and centrifugal forces during machine operation.